## Galvanometer is used r the radius of the coil in centimeter

6, the angle of the needle deflection in degrees; and I, the current in c.g.s. units.

FIG. 1—The tangent galvanometer consists essentially of a short magnetic needle suspended at the center of a coil of large diameter and small cross-section. In practice the diameter of the coil is approximately 17 times the length of the needle. If the instrument be so placed, that when there is no current in the coil, the suspended magnet lies in the plane of the coil, that is, if the plane of the coil be set in the magnetic meridian, then the current passing through the coil is proportional to the tangent of the angle by which the magnet is deflected from the plan of the coil or zero position, hence the name: "Tangent galvanometer."

When the tangent galvanometer has more than one turn the factor 2% should be multiplied by the number of turns (N) on the coil.

Example.—Find the current in a tangent galvanometer having a coil of 25 centimeter radius, when the needle deflects an angle of 10 degrees. Assume H=0.2066 FIG. 1—The tangent galvanometer consists essentially of a short magnetic needle suspended at the center of a coil of large diameter and small cross-section. In practice the diameter of the coil is approximately 17 times the length of the needle. If the instrument be so placed, that when there is no current in the coil, the suspended magnet lies in the plane of the coil, that is, if the plane of the coil be set in the magnetic meridian, then the current passing through the coil is proportional to the tangent of the angle by which the magnet is deflected from the plan of the coil or zero position, hence the name: "Tangent galvanometer."

25X0.2066 2X3.1416

X0.1763 from which

If it be desired to determine the tangent galvanometer constant for a certain location, the current through the galvanometer may be measured by inserting an instrument of known accuracy in the circuit.

With the circuit of known value the galvanometer constant can be computed from the following formula as follows:

This type of galvanometer however, has a number of disadvantages of which the following are the most prominent.

1. The coil being much larger than the needle, and hence far away from it, reduces to a considerable extent its sensitiveness.

2. The readings are effected by an external magnetic field which may exist in the neighborhood of the instrument.

3. Accuracy of readings may be lessened by changes in the earth's magnetic field, which changes may be of considerable proportions especially during magnetic storms.

D'Arsonval Galvanometer.—In this type of galvanometer the aforementioned disadvantages are largely eliminated.

The principal design is shown in figs. 2 and 3. In this instrument the indicating needle is attached to a coil of wire, through which the current flows, and inside of which is an iron core.

. The coil is free to turn with the pre which is held in place with a pin, and suspended between the poles of a horseshoe magnet. When the current to be measured flows through the coil, a magnetic field is set up in and around it, causing the coil to turn.  FIGS. 2 and 3—Drawing showing essential features of construction and principle of operation of D'Arsonval type galvanometer.

This rotating tendency is prevented by the twisting of the wire which suspends the loop. By planning the weight of the wire used, the number of turns in the coil and the amount of resistance used, this galvanometer can be used for determining small amounts of currents. It is on this principle that many commercial types of current measuring devices is based.

The readings of the galvanometer may be facilitated by means of a mirror which is usually attached to the coil in such a way that a beam of light from a light source directed to the coil by a lense system, will be reflected back on to a semicircular graduated scale placed at a suitable distance from the mirror as shown-in fig. 4. In this way a small deflection of the coil and mirror will produce an enlarged amplification of the beam of light on the scale, which reading may be accurately accomplished by means of a telescope.

Electrochemical Current Measurements.—Another method of current determination is by utilizing the amount of the chemical decomposition the current causes in passing through the electroylete in a cell. FIG. 4a—Weston model 440 movable coil galvanometer.

If a constant current of (I) amperes is passed through the electrolyte during (0 seconds, the weight increase on the cathode in milligram CM) is direct proportional to the amount of current multiplied by the electrochemical equivalent of the metal (C) and the time of current flow.

M=CX/X* For 7 = 1 ampere and t = 1 second; M = C. Among the various apparatus based on the electrochemical effect of an electric current are: 